• Use of threshold extension demodulators in some cases (FM systems only)

• Use of forward error correction on many digital systems, and above 10 GHz with interleaving to mitigate rainfall fading

Now let us trace a signal through the communication subsystem typical of Figure 15.32. On the transmit side the FDM baseband is fed from the multiplex equipment through the baseband patch facility to the modulator. A spreading waveform is added to the very low end of the baseband to achieve constant loading. The baseband signal is then shaped with a pre-emphasis network (see Section 2.6.5). The baseband so shaped frequency modulates a carrier, and the resultant is then upconverted to a 70-MHz IF. Patching facilities usually are available at the IF to loop back through the receiver subsystem or through a test receiver for local testing or troubleshooting. The 70-MHz IF is then fed to an upconverter, which translates the IF to the output frequency (6 or 14 GHz). The signal is then amplified by the HPA, filtered by a low-pass filter, directed to the feed, and radiated by the antenna.

For reception, the signal derives from the feed and is fed to a low-noise receiver. In the case of an INTELSAT Standard A earth station, the low-noise receiver looks at the entire 500-MHz band (i.e., in the case of 4-GHz operation, the band from 3700 to 4200 MHz), amplifying this broadband signal 20-40 dB. When there are long waveguide runs from the antenna to the equipment building, the signal is amplified still further by a low-level TWT or SSA called a driver. The low-noise receiver is placed as close as possible to the feed to reduce ohmic noise contributions to the system. GaAs and PHEMT LNAs have noise temperatures between 30 and 50 K and 25-35 dB gain.

The comparatively high-level broadband receive signal is then fed to a power split. There is one output from the power split for every down-converter-demodulator chain. In addition, there is often a test receiver available as well as one or several redundant receivers in case of failure of an operational receiver chain. It should be kept in mind that every time the broadband incoming signal is split into two equal-level paths, there is a 3-dB loss due to the split, plus an insertion loss of the splitter. A splitter with eight outputs will incur a loss of something on the order of 10 dB.

A downconverter is required for each receive carrier, and there will be at least one receive carrier from each distant end. Each downconverter is tuned to its appropriate carrier and converts the signal to the 70-MHz IF. In some instances dual conversion is used.

The 70-MHz IF is then fed to the demodulator on each receive chain. The resulting demodulated signal, the baseband, is reshaped in the de-emphasis network (see Section 2.6.5) and spreading waveform signal is removed. The resulting baseband output is then fed to the baseband patch facilities and thence to the demultiplex equipment.

15.5.1.1.1 Digital Operation. Present-day configurations of digital earth stations are very much like their analog counterparts. FM modulators/demodulators are replaced by digital modulators/demodulators. The modulation is BPSK or QPSK. Heavy bit packing radios such as encountered in line-of-sight microwave are as yet to be applied to satellite links. Forward error correction is commonly applied, often with convolutional coding rate